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I'm pretty new to cross myself, I just picked up a used Empella Bonfire, I've got 36/48 in the front and 12-25/27 in the rear... I've seen alot of pros (Nys) running a single chain ring in the front at 42 teeth...

My crank arms are 172.5 just like my road bike

If you're new to cross, I'd stay away from the carbon cranks (for now anyway) no sense in getting all that nice carbon all scratched up!

hello mads, welcome to cyclocross!
first: if racers run a single chainring, it is usually 42 tooth, sometimes 44 tooth. 11-23 is fine for most of the courses, but 12-25 is fine as well. i don't really like 12-27 for racing because of the 3 t-jumps from 21 to 24 and from 24-27. if you use your cross bike for light bike tours, i would rather go for a double chainring setup, because there will be longer uphills and you don't want to go full speed on them all the time just to be able to pedal. in races 39-46 or 39-48 is normal, chainrings over 48 do not make sense, also because if you jump over obstacles, you can bang the chainring onto the obstacle.
i use the same crank arm length as on the road bike(172,5 mm), but if you are used to 175 from the mtb, i would choose 175 mm for cross as well, with the disadvantage that your shoes are on the ground in narrow corners earlier, if you start to accelerate(same point as for road criterium bikes). if you're used to mtb, i would then also go for 39-48 with 12-27, or a compact crank with 36 inner chainring. too much difference between the chainrings is bad because f chain tension, if you ride singletrails with roots in the small chainring the chain can easily fall off(deda or other plastic parts are a tip, think they're also calles 3rd eye or dogfang)you have to try if mtb cranks work with the frame(because the chain stays are wider apart for the tire clearance), but i would not do that. a cross bike is much faster than an mtb because of the 28 inch wheels, you won't use the small chainrings.
i have dura ace cranks on my cyclocross bike, i had to modify a little bit. i placed a shimano 9 speed cassette spacer between the right bearing and the frame to get the crank further outside, because the chainring was scratching on the frame. i think that mtb cranks come with those kind of spacers, but it was sunday evening and i had to build the bikes together...

simon wrote:i would choose 175 mm for cross as well, with the disadvantage that your shoes are on the ground in narrow corners earlier, if you start to accelerate(same point as for road criterium bikes).

simon, everything you say makes perfect sense. But since most modern CX races are off-road criteriums, some bikes have higher bbs just like the special criterium bikes on the road. My OCCP is built slightly higher than my old Empellas and I love it. Sure, often an Empella feels "smaller" or "more nimble" but when pedaling through turns or coasting and accelerating hard on the exit you have the clearance between pedals and ground that you need.

hello mads
no problem, that's the sense of a forum...
i tried mtb front derailleurs, because many cross frames have the cable down the seattube and the cable goes around some kind of pulley to direct it 180°, then normal road fd's are used. the problem is that this pulley is a magnet for mud, grass, etc.
but i never got to a point where mtb fd's worked right. they're usually made for 42 t chainrings, and with 46 or 48 they don't really work(you can bend them with hammer or so, but that's not really the way to treat expensive parts). there is a little metal piece to convert a normal road derailleur that it functions with the cable coming directly from the top. it is a little bit hard to explain, but it is the best solution. if you're interested, i can try to get you one of them.

simon wrote:i tried mtb front derailleurs, because many cross frames have the cable down the seattube and the cable goes around some kind of pulley to direct it 180°, then normal road fd's are used. the problem is that this pulley is a magnet for mud, grass, etc.but i never got to a point where mtb fd's worked right. they're usually made for 42 t chainrings, and with 46 or 48 they don't really work

If you get a XTR front derailler of 95x or previous series they are designed 48t chainrings. The current 96x ones are designed for 44t rings.

The new road derailleurs designed for compact cranks should also work well.

simon wrote: there is a little metal piece to convert a normal road derailleur that it functions with the cable coming directly from the top. it is a little bit hard to explain, but it is the best solution. if you're interested, i can try to get you one of them.

Simon, do you happen to have a picture of this thing? I'm not really concerned about the extra weight of the cable and the turning wheel as my crossbike isn't much of an ww bike, but it sounds like a good add-on to make shifting better in muddy conditions.
Would it work with campy shifters and fd?

i've just decided to have one of my three bikes with 39-46, so i'm waiting for my converter(don't have a better name for it). i hope to get it next weekend, then i will make pics of it and put it on a scale. i think the weight of this piece is nearly the weight that you lose by dismounting the pulley and shortening the cable.

ridley x-night. have a look at the new cyclocross season topic, there is a pic of one of them. i hope to have some time left, then i will make a few detailed pics and a spec list with weights.
if you race, you need at least 2 bikes, better 3. and off course, several pairs of wheels with different gears and tubulars.

Well you havn't even listed the track bike, SS, fixed gear road bike and TT bike yet, maybe time to get saving

simon wrote:i've just decided to have one of my three bikes with 39-46, so i'm waiting for my converter(don't have a better name for it). i hope to get it next weekend, then i will make pics of it and put it on a scale. i think the weight of this piece is nearly the weight that you lose by dismounting the pulley and shortening the cable.

Can you please discribe this in more detail please? I'm not sure what you are describing.